This is my concern. It's probably going to be mostly handheld I expect
The same was said when I was getting my macro lens.. You should use a tripod
Almost all my macro pics are hand held
Really if you care about image quality then you should use a tripod whenever you can and conditions permit (sometimes in dynamic situations this is not possible). Most wildlife situations you can always use a tripod, or in fact always should. You know a lot of wildlife photography is sitting and waiting, for birds that often means in a hide for a few hours.
You don't want to be holding a big lens anymore than you have to. Much nicer to sit it down on a tripod and leave your arms and hands free to adjust the zoom, composition, exposure, focus, etc. I can guarantee you that even if you don't have a tripod you will have a monopod to take the weight of things. Not least having the lens attached to a monopod or tripod helps carrying it so much easier, just sling it over your should. You really don't want such a lens hanging on your camera neck strap.
It is also very hard to compose a photo on a very long lens. the slightest moment completely disrupts the balance of the scene. IS will help preserve sharpness but it wont guarantee the composition as the lens adjusts.
Then there is the issue that unless you are photographing a predatory bird then it will be moving and fidgeting a lot so you will want a fast aperture anyway, something IS wont help with. 1/500th is a safe bet for small birds, so not really too different to what you would want hand held anyway. You might getaway with slower shutter speeds but it all depends on the bird, the exact situation and luck. On the other hand a Heron will be absolutely motionless so you could get away with a much slower shutter speed and IS could be very handy.
Birds in flight is one situation when it is sometimes nice to go without a tripod and you would think IS would help, but again you need a fast shutter speed to limit motion blur of falling wings etc., AND you need to be careful when panning using IS. Most lens detect horizontal panning quite well as used in sports, but many don't detect vertical panning at all, or as well, and then there are plenty of situations when the panning is more diagonal or changes and the IS system wont keep up. Lots of factors here but if at all possible i would choose fast shutter speed, IS would be a backup if I absolutely cannot push my ISO any further.
Then there is the fact that IS can negatively impact image quality, especially out of focus rendering. It also has a tendency to improve sharpness massively from being a useless blur to being reasonably sharp but often it fails to hit the critical sharpness that a tripod mount can give (this does vary by lens).
Now, if someone could magic IS/VR into my 300mm f/4.0 for no charge absolutely 100% I would take it and occasionally make use of it. No doubt it can be a big help under non-ideal situations (low light and you left your tripod behind). But it isn't a deal breaker and other factors like focus, sharpness and length have a bigger impact.
I think you need to go to a shop and try out all lens. See if a 300mm IS with a 1.4xTC focues as you would want, see if the 400mm prime works OK outside in average lighting. feel the weight of these lenses and then think if you want to go several hours with a lens like this around your neck, or properly supported.
Finally, I know i have been pushing you away from the 100-400mm. The 100-400 is a good lens and can take great photos so if you do decide that works best for you then that is great, don't worry about it. The thing is you will be using that lens at 400mm almost the whole time for wildlife so that is what you need to compare it against. Lastly, if you really want a casual walkabout telephoto lens with no need for a tripod then look at a 70-300mm, much smaller and lighter. Not quite as long but you can crop and loose a little IQ, the thing is if you are not going to use a tripod then you have already made the tradeoff between convenience and IQ so the 70-300 will be along the same lines, and is a much easier lens to hand hold.
Whatever you do I would try to go second hand, both the 100-400 and 400 prime will get replaced soon, so try to save your money.
